The present invention relates to a method for automatic determination of exposure of a radiographic film in a diagnostic radiology installation in which the radiographic film cooperates with an intensifying screen. The invention is also concerned with an automatic film-exposing device for the utilization of said method in a diagnostic radiology installation in which provision is made for a radiographic film and an intensifying screen exposed to x-radiation.
In order to ensure that the image of a subject or object formed on a radiographic film can be processed under optimum conditions, the different points constituting this image must display sufficient contrast with respect to each other. In other words, blackening of the radiographic film must be correct from one radiograph to the next in spite of any differences in opacity which may be exhibited by the object under radiographic inspection.
Blackening of the film is related to its exposure or so-called radiant exposure, which is the product of the dose rate, or radiation intensity to which the radiographic film is subjected, and the time during which the film is exposed to said radiation.
As a general rule, there are two methods in use for exposing radiographic films in diagnostic radiology installations:
(1) In the first method, the radiographic films are exposed directly to the x-rays which have passed through the object under radiographic inspection.
(2) In the second method, the films are exposed mainly to light emitted by an intensifying screen. By means of this screen, the x-rays which have passed through the object under radiographic examination are converted to light by a fluorescence phenomenon. It is worthy of note that diagnostic radiology installations in which a radiographic film cooperates with an intensifying screen offer a very important advantage from the point of view of the patient who is to undergo an examination. This advantage lies in the fact that the exposure time required in order to obtain an image is much shorter (by a factor of the order of 10) than the time which is necessary when using radiographic films exposed directly to the x-radiation, with the result that the radiation dose received by the patient is reduced in the same ratio.
A certain number of diagnostic radiology installations which utilize the first method are equipped with an automatic film-exposing unit, thus making it possible to obtain automatically the exposure corresponding to correct blackening of the radiographic film (which is exposed directly to the x-radiation).
As a rule, an automatic film-exposing unit of this type incorporates a detector which is sensitive to x-radiation and delivers a current proportional to the dose rate of x-radiation which passes through the radiographic film. The detector which is sensitive to x-radiation is usually constituted either by a conventional ionization chamber or by a semiconductor cell or else by a screen for converting the x-ray photons to light which is guided to a photomultiplier, the light being then converted by said photomultiplier to an electric current I which is proportional to the dose rate. Irrespective of the type of detector employed, the current I which is proportional to the dose rate is integrated over time from the initial instant of the period of exposure and there is thus obtained a value which increases with time and represents the integrated exposure of the film. This increasing value is compared during the period of exposure with a fixed reference value which has been established beforehand as a function of the characteristics of the film. The exposure time is controlled so as to end at the instant when the comparison indicates that the value which is representative of the exposure is equal to the reference value.
Thus, supposing that the dose rate of the current I during the exposure time T is constant, the degree of exposure of the film or radiant exposure EX is equal to the product of the dose rate I and the exposure time T, namely: EQU EX=I.times.T
In this configuration in which the radiographic film is directly subjected to the x-radiation, constant blackening of the film is obtained from one exposure to the next and independently of the exposure time on condition that the product of the exposure time T and the dose rate I is constant. The radiographic film which is directly exposed to the x-radiation complies with the so-called reciprocity law.
The reciprocity law indicates that the optical density of the film is determined by the integrated exposure and is independent of the length of time during which this exposure is made.
On the other hand, in the case of pairs consisting of intensifying screen and radiographic film, this reciprocity law is no longer verified and there is an observable departure from the reciprocity law which tends to decrease blackening (darkening) of the film when the exposure time T has been increased in order to maintain the product I.times.T at a constant value. This arises, for example, from one radiograph to the next when examination requirements have dictated the need to utilize a different x-ray energy level at which it is necessary to reduce the dose rate or in other words the value of the current I.
This deviation from the reciprocity law which may vary according to the type of film represents the relative variation in the radiant exposure required in order to obtain a constant optical density when the exposure time T varies. For example, in the case of a radiant exposure EX=I.times.T=1 with T=0.1 second, in order to obtain correct blackening of the film, the radiant exposure EX must increase to about 1.3 in respect of an exposure time T=1 second and must increase to approximately 2 in the case of an exposure time T=4 seconds.